The Tillmans accuse the Army of lying and conducting improper investigations

Patrick Tillman says his son may have been deliberately killed

(CNN) -- Six years removed from the death of their son -- former NFL star Pat Tillman -- in Afghanistan, Patrick and Mary Tillman say they haven't gotten the full truth from the U.S. Army and don't expect to.

"No, we don't know the whole truth," Mary Tillman said on CNN's "Larry King Live" Monday night. "They never did a criminal investigation after Pat was killed. So, without that proper criminal investigation, they destroyed evidence, which is long gone. There really is no way for us to know exactly what happened to him."

Tillman was a safety with the Arizona Cardinals in the National Football League when he passed up a multimillion-dollar contract to join the Army in 2002, after the September 11, 2001, attacks on New York and Washington.

He was a corporal with the service's elite Rangers when he was killed during an ambush in an Afghan mountain pass in April 2004.

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The Army posthumously awarded Tillman the Silver Star, the third-highest U.S. combat decoration, announcing that he had been killed leading a counterattack. But it later admitted that officers in his chain of command knew almost immediately that he had been shot accidentally by his own comrades.

For five weeks, his family was not told that the death was a result of friendly fire.

Former U.S. Defense Secretary Donald Rumsfeld and retired Gen. Stanley McChrystal, former commander of U.S. forces in Afghanistan, both declined to appear with the Tillmans on CNN. However, the U.S. Army did issue a statement.

"As an Army, we failed in our duty to the memory of a fallen soldier and to his family. The failures of a few brought discredit to the Army and compounded the grief suffered by the Tillman family," the Army statement said. "The Army truly regrets the pain and suffering endured by the Tillman family as a result of this tragic, friendly fire accident and the shortfalls in reporting accurate information to them in the days and weeks after Pat's death."

Mary Tillman took issue with the Army's characterization of events.

"They weren't shortfalls. They weren't missteps and they weren't errors," Tillman said. "They were deliberate attempts to cover up what happened in order for them to use Pat's death for propaganda purposes at a time during the war in 2004 when [the] Abu Ghraib Prison scandal was breaking ... it was a terrible time for the military and for that administration, and Pat's death was an opportunity for them."

Patrick Tillman said he hasn't ruled out that his son may have been deliberately killed.

"I haven't eliminated it," he told King. "There is strong evidence of two shooters, not one."

The Tillmans, who are divorced, are promoting this week's opening of "The Tillman Story," a new documentary about the death of their son and the controversy that followed it.

Mary Tillman also is the author of the book "Boots on the Ground by Dusk: My Tribute to Pat Tillman."